The dimerization of functionalized olefins is an important chemical reaction, which allows the efficient production of unsaturated compounds or building blocks having multiple functional groups.
The dimerization of functionalized olefins, e.g. Michael acceptors, is generally known in the state of the art. Typically, transition metal complexes such as Ni-, Ru-, Rh- or Pd-complexes are used to catalyze this reaction.
DE 3336691 for example discloses a process for the nickel catalyzed dimerization of acrylic acid derivatives to yield linear unsaturated dicarboxylic acids and the use of these dimethylcarboxylic acid derivatives as monomers and/or co-monomers in poly-mer, polycondensation and hydration reactions.
U.S. Pat. No. 4,451,665 discloses a process for dimerizing a lower alkylacrylate or a lower alkyl-methacrylate in the presence of a palladium (II) catalyst to yield mixtures of isomeric linear dimers. In particular, this reaction was used for the preparation of mixtures of linear isomeric dimethyl hexenedioates and dimethyl 2,5-dimethylhexenedioates.
The dimerization products are typically obtained as mixtures of several individual isomers. The overall yields of the dimerization products are usually limited due to the formation of side products.
It is further known in the state of the art that the selective tail-to-tail dimerization of functionalized olefins can be achieved using N-heterocyclic carbenes (NHCs) as the catalyst providing the dimerization products in good yields and selectivities.
Biju et al., Angew. Chem. Int. Ed. Engl. 2011, Vol. 50(36), pp. 8412-5, disclose a process for the coupling of two activated olefins in the presence of an N-heterocyclic carbene catalyst (NHC-catalyst) and the organic base 1,8-Diazabicyclo[5.4.0]undec-7-en (DBU). Specifically, the dimerization reactions are performed in 1,4-dioxane using 10 mol-% of protonated NHC-catalyst as a perchlorate salt and 1.0 equivalent of DBU. The reaction mixture is heated to 80° for 24 hours. The process of Biju et al. allows the selective production of several linear dimerization products in moderate to excellent yields and high E/Z ratios (up to 97:3).
Matsuoka et al., Org. Lett. 2011, Vol. 13(14), pp. 3722-5, disclose a process for the selective tail-to-tail dimerization of substituted acrylates, in particular of methyl methacrylate, in the presence of an N-heterocyclic carbene catalyst. Specifically, the dimerization reactions are performed in the absence of any solvent or in the presence of toluene using 10 mol-% of free NHC-catalyst and by heating the reaction mixture to 80° for 8 or 24 hours, yielding the linear dimerization products in good to excellent yields and E/Z ratios in the range of 88:12 to 98:2.
The reaction mechanism of the NHC-catalyzed tail-to-tail dimerization of substituted acrylates was later identified by the same authors and is described in Kato et al., J. Org. Chem. 2013, Vol. 78(17), pp. 8739-8747.
Kato et al., J. Org. Chem. 2014, Vol. 79(10), pp. 4484-4491, disclose a process for the tail-to-tail dimerization of methacrylonitrile as well as the co-dimerization of methacrylonitrile with n-butyl methacrylate in the presence of an N-heterocyclic carbene catalyst and an alcohol additive. Specifically, methacrylonitrile was dimerized either in the presence of 5 mol-% of an NHC-catalyst and 5 to 50 mol-% of a lower alkyl alcohol or an aromatic alcohol as additive, or in the presence of a mixture of 5 to 50 mol-% of a lower alkyl alcohol and 0.5 to 1 mol-% of an aromatic alcohol as additive. The reactions were carried out in bulk or by adding an external solvent such as 1,4-dioxane. Their experiments reveal that aromatic alcohols, such as 2-naphthol or hydroquinone, are significantly less suitable to accelerate the investigated dimerization reactions, than lower alkyl alcohols, such as isopropanol or n-butanol. The authors further report that, in contrast to the dimerization of methacrylonitrile, the addition of an alcohol additive to the NHC-catalyzed dimerization of n-butyl methacrylate has detrimental effects on the reaction rate and yield.
However, long reaction times and large amounts of catalyst are generally needed to achieve high yields and selectivities. Thus, there is a need to further improve the NHC-catalyzed selective tail-to-tail dimerization of functionalized olefins.